Size–resolved mixing state of ambient refractory black carbon aerosols in Beijing during the XXIV Olympic winter games (2023)


Black carbon aerosols (BC), as products of incomplete combustion activities (e.g., diesel engines, residential solid fuel, open burning), have attracted widespread interest because they not only have an impact on climate and the environment but also have a negative impact on human health (Cassee et al., 2013; Peng et al., 2016). BC emissions in East Asia are among the highest in the world (Bond et al., 2013), and Beijing, as a mega-city in East Asia, is not to be underestimated in terms of BC emissions from transportation, industry, and residential use (Zhang et al., 2013). In addition, the aging process of BC is complicated by the complex air pollution caused by the rapid urbanization and the increase in vehicles in Beijing (Liu et al., 2019a, 2020a). Much research in recent years has focused on the mixing state of BC, such as coating thickness and aerosol components mixed with BC (Liu et al., 2019a; Xie et al., 2020). However, few researchers have addressed the problem of the relationship between morphology, coating thickness, and coating composition. The morphology of BC has a great influence on the assessment of its absorption properties and radiative forcing (Wang et al., 2021). It is worth noting that during major events, air quality tends to be better in Beijing and surrounding areas due to the strict control measures (Guo et al., 2013; Sun et al., 2016). The detailed characterization of the mixing state of BC in different environments is essential for understanding the aging of BC in the atmosphere.

Studies based on laboratory and field observations have shown significant differences in particle size from different types of aerosols and different emission sources (Fennelly, 2020; Fröhlich-Nowoisky et al., 2016; Li et al., 2016); for example, coarse particulate emissions from coal combustion are generally higher than those from biomass (Yang et al., 2021). There are also differences in the harmful systemic health effects of particles at different aerodynamic diameters (Dae) (Kwon et al., 2020); for example, ultrafine particles (PM1.0) retained longer in the lung than fine particles (PM2.5) (Schraufnagel, 2020). It has been shown that the morphology, optical properties, coating thickness, hygroscopicity and coating composition of BC-containing particles with different particle sizes are very different (Liu et al., 2020b; Wu et al., 2019a; Xie et al., 2020). Therefore, the characterization of size-resolved BC is crucial for understanding the health effects of BC, improving the assessment of its absorption-enhancing effects, and quantifying its impact on climate.

There are two main types of technical methods used to study size-resolved BC morphology and mixing state. One of the methods is to directly observe the morphology of a single BC using electron microscopy or atomic force microscopy (Li et al., 2016), but it cannot meet the demand of real-time, continuous, and high-frequency online analysis. Another technique is to use a single-particle soot photometer (SP2) in tandem with other particle size screening instruments (DeCarlo et al., 2004; Liu et al., 2019b, 2020b), which can overcome the shortcomings of the former. Many researchers have used a differential mobility analyzer (DMA) and SP2 to characterize BC morphology and mixing state (Han et al., 2019; Hu et al., 2021; Wu et al., 2019b; Zhang et al., 2018a). However, the method is limited by the way the DMA selects the particle diameter, i.e., the electrical mobility related to both the mobility and aerosol charges, which can allow unwanted mobility diameter aerosols to be screened out (Wang and Flagan, 1990; Wu et al., 2019b). Recently, an aerodynamic aerosol classifier (AAC) has been introduced to replace the DMA in tandem systems (Liu et al., 2020b; Yu et al., 2022), selecting aerosols with a certain aerodynamic diameter and thus avoiding the effect of aerosol charge. This is of great significance for understanding both the BC morphology and the mixing state in the real atmosphere and quantifying the direct or indirect climate effect of aerosols.

BC, i.e., freshly emitted BC mixed with other compounds in the atmosphere through the aging process to become coated BC. During aging, the morphology, coating thickness, coating composition, and absorption ability will change. Generally, the more aged BC is, the thicker the coatings, the higher the hygroscopicity, the more regular the morphology, and the stronger the absorption enhancement (He et al., 2015; Peng et al., 2016). Based on the developed instruments, it is now established from a variety of studies that particle morphology is commonly described by the dynamic shape factor (DeCarlo et al., 2004). Liu et al. (2020b) found that the dynamic shape factor of refractory black carbon-containing particles (rBC) decreased from 1.43 to 1 and that the coating thickness of rBC increased from 1.67 to 2.84 as Dae increased from 200 to 700nm. This means that as the Dae of rBC increases, rBC becomes progressively more regular in morphology and tends to be spherical. The effective density of rBC increased as the dynamic shape factor decreased (Liu et al., 2019b). Wu et al. (2019a) found that an increased inorganic fraction and more oxidized organic coatings usually determine a thicker coating thickness and larger rBC size. Overall, there are relatively few studies on the microphysical properties of size-resolved rBC, particularly in relation to the morphology, mixed material, and coating thickness of size-resolved rBC.

A previous study (Liu et al., 2020b) characterized the coating thickness, optical properties, and morphology of size-resolved rBC in Beijing using an AAC-SP2 tandem method. To explore the size-resolved coating material, we performed a novel online measurement system by coupling an aerodynamic aerosol classifier (AAC) with different aerosol measurement instruments, including a single-particle soot photometer (SP2) and a single-particle-aerosol mass spectrometer (SPA-MS). Several studies have been carried out to characterize the coating composition of BCc using SPA-MS (Sun et al., 2022; Xie et al., 2020). It is worth noting that this measurement was conducted at an urban site during the XXIV Olympic Winter Games with relatively clean air conditions in Beijing. Measurement of the size-resolved mixing state of rBC illustrates the difference between irregular and regular rBC. In addition, we conducted a detailed analysis of the mixing state, absorption ability and morphology of rBC at special time points during the Winter Olympics and under the influence of different source air masses. These results enhance the understanding of the climate and environmental effects of different rBC types under different pollution conditions and provide morphology-composition-coating thickness relationships that can be used to constrain climate and air quality modeling.

Section snippets

Sampling site

The measurement of rBC was performed from 10 February to 1 March 2022at the State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry (LAPC, latitude: 39.97°N, longitude: 116.37°E). The sampling site is located between the North 3rd and 4th Ring Road of Beijing and is adjacent to the Jingzang highway. It is less than 2km away from Beijing's Bird's Nest Stadium, the venue for the opening ceremony of the XXIV Olympic Winter Games (Fig. 1b). Transportation emissions

Overview of the field observations

Fig. 2 and Fig. S6 show the time series of meteorological parameters and pollutants measured at the LAPC site during the campaign. The average mass concentration of PM2.5 during the observation period was 28μgm−3, which is 48μgm−3 lower than the average concentration from 2014 to 2022 and is the lowest concentration in the last 8 years (Fig. 1e). This may be related to the control measures implemented in Beijing between January and March 2022. As illustrated in Fig. 2, according to

Conclusions and implications

A novel tandem system consisting of an AAC, a SP2, and a SPA-MS was conducted to explore the aerodynamic size-resolved coating thickness, light absorption properties, and coating components coated on the rBC core of rBC-containing particles in Beijing during the XXIV Olympic Games. Two aerodynamic size (Dae=200nm and Dae=300nm) rBC particles were selected during the whole observation to study the effect of relative differences in morphology on the coating thickness, coating compositions,

CRediT authorship contribution statement

Yuting Zhang: Conceptualization, Experiment, Methodology, Software. Hang Liu: Investigation, Experiment. Shandong Lei: Investigation. Aodong Du: Investigation, Software. Weijie Yao: Investigation. Yu Tian: Investigation. Yele Sun: Investigation. Jinyuan Xin: Investigation. Jie Li: Investigation. Junji Cao: Resources. Zifa Wang: Resources, Supervision. Xiaole Pan: Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.


This study was supported by the National Key Research and Development Program of China (No. 2022YFC3701000, Task 4).

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